Access to auxiliary images

Hi Kiril,

I see in the readme that it is possible to convert multiple images. However, does heic-convert also have a way to access embedded auxiliary images (e.g., Portrait photo depth map) as the non-JS version of heif-convert (the example app from libheif repo) does?

In other issues I saw you mention that heic-convert is based on the JS version of libheif. When checking [heif_enscripten.h](https://github.com/strukturag/libheif/blob/master/libheif/heif_emscripten.h#L232) I see some references to auxiliary images, but I can't tell if the methods are actually implemented.

Many thanks in advance.